No search was conducted on the subject matter of this specification in the U.S. Patent and Trademark Office or any other search facility. We are unaware of any prior art which is relevant to the method of reducing the green density of a slip cast article as disclosed and claimed in this specification.
Silicon metal particles and yttrium containing particles have been mixed and formed into slips so that articles may be slip cast therefrom. The purpose of using silicon particles is that these particles may be subsequently nitrided in order to form silicon nitride, a ceramic material which has excellent thermal resistance properties. Yttrium is used in casting such articles because the yttrium serves as an excellent sintering aid in a sintering operation carried out on the formed article once the silicon particles have been transformed into silicon nitride. Thus, the order of progress is one in which an article is slip cast, the so-formed article being formed of silicon particles and yttrium containing particles. The so-formed article is then subjected to a nitriding operation in which the silicon particles are transformed into silicon nitride. Thereafter, the nitrided article is subjected to a sintering operation in which the yttrium acts as a sintering aid in permitting the silicon nitride particles to form into a uniformly high density silicon nitride article having excellent thermal resistance characteristics. The arts of slip casting, nitriding, and sintering are all well known in the art and no further discussion thereof will be contained herein.
One difficulty we have had in forming slips of silicon metal particles and yttrium containing particles comes about in that the slips generally have a relatively poor stability when slip cast as well as a relatively short shelf life. By poor stability we mean that the formed slip does not have uniform chemical and rheological stability for prolonged periods of time necessary to cast articles having some degree of thickness. In fact, we have found that slips of silicon metal particles and yttrium containing particles are relatively unstable and generally have a very short period of time in which they may be slip cast, the time in some instances being only one-half hour to one hour. In most cases a minimum of 4-8 hours of a stable slip is required in order to cast that slip in a slip casting process to form an acceptable slip cast part. By poor shelf life we mean that chemical properties of the slip change as the slip sits on a shelf between times of using the slip in a casting operation.
In general, the yttrium containing particles forming the yttrium containing sintering aid are formed from a compound which has at least some solubility in water to form Y.sup.+3 ions. These Y.sup.+3 ions have a high potential for totally flocculating the silicon metal particles into a semiporous solid. Thus, in a situation where yttrium containing particles are mixed with silicon metal particles to form a slip, the yttrium containing particles normally dissolve in water to form the Y.sup.+3 ions. Once a significant number of these ions have formed, the ions are effective to totally flocculate the silicon metal particles into a semiporous solid which in effect sets up the material which is to be used as the casting slip and thereby ends the castability of that slip. The concentration of the yttrium containing particles in the slip determines the amount of time before the flocculation of the silicon particles takes place. In any reqard, the flocculation of slip compositions we use containing silicon metal particles and yttrium containing particles usually occurs within one-half to one hour after that slip has been cast into a mold. This time is too short to cast an article having other than a very small thickness dimension.
In an application entitled "Method of Forming A Relatively Stable Slip of Silicon Metal Particles and Yttrium Containing Particles", U.S. Ser. No. 512,095, filed on even date herewith, we described a method by which a fluoride salt was added to the casting slip to render Y.sup.+3 ions ineffective in flocculating silicon metal particles. The slip which resulted from this teachings of the method of our other application was one that would produce a green slip cast article having a green density of about 1.60 g/cc with a packing fraction of about 0.65 to 1.70 g/cc with a packing fraction of about 0.69. At this particular green density, the ability to nitride the finished article was such that normally it was difficult to nitride relatively thick sections of the cast article.
We determined that it would be best to have a green density in the range of 1.42 to 1.55 with a packing fraction in a range of 0.58 to 0.63 if one desired to manufacture thick sections of a nitrided material. The lower green density permitted better penetration of the green body for the purpose of nitriding the same.
It is therefore a principal object of the method of this invention to provide a method of reducing the green density of an article cast in a slip casting operation whereby the article is thereby easier to nitride in a subsequent nitriding operation.